59 



AbStraCt.-Individuals of the tropi- 

 cal loliginid squid Photolohgo sp. 1 were 

 collected from the tropical waters of the 

 northwest continental shelf of Austra- 

 lia. Both males and females exhibited 

 pronounced phenotypic variation in size 

 at maturity. Statolith increment analy- 

 sis was carried out to determine indi- 

 vidual age. On the basis of the assump- 

 tion that statolith increments were de- 

 posited daily, counts revealed that this 

 species had a short life-span and that 

 all individuals were younger than 160 

 d and exhibited linear growth over the 

 size range sampled. All of the longest 

 squid collected were females, which 

 achieved this size disparity predomi- 

 nantly by being older than males rather 

 than by growing at a faster rate. Age 

 analysis revealed that small mature 

 individuals were considerably younger 

 than large mature individuals. A large 

 size distribution of mature individuals 

 was therefore achieved by variation in 

 age at maturity. Possible causal mecha- 

 nisms are considered. 



Variation in size and age at 

 maturity in Photololigo 

 (Mollusca: Cephalopoda) from the 

 northwest shelf of Australia 



George D. Jackson 



Department of Zoology, University of Western Australia 

 Nedlands, Perth. Western Australia, 6007 



Julia Yeatman 



Department of Marine Biology James Cook University of North Queensland 

 Townsville, Queensland 48 11, Australia 



Manuscript accepted 12 September 1995. 

 Fishery Bulletin 94:59-65 ( 1996). 



Pelagic cephalopods are charact- 

 erized by fast growth, early matu- 

 rity, and predominantly indetermi- 

 nate growth (Rodhouse and Hat- 

 field, 1990; Alford and Jackson, 

 1993). This is especially pronounced 

 in tropical species (Jackson and 

 Choat, 1992). Cephalopods also 

 show considerable variation in in- 

 dividual growth rates (e.g. Nat- 

 sukari et al., 1988; Jackson, 1989; 

 Arkhipkin and Mikheev, 1992; 

 Villanueva, 1992). Furthermore, a 

 marked plasticity has been noted in 

 size at maturity for a number of 

 cephalopods (e.g. Lolliguncula breuis, 

 Hixon, 1980; Loligo opalescens, 

 Hixon, 1983; L. pealei, Macy, 1982; 

 Alloteuthis subulata, Rodhouse et 

 al., 1988; Dosidicus gigas, Nesis, 

 1983; Sepia officinalis, Boletzky, 

 1983; and Sepia pharaotiis. Dun- 

 ning etal., 1994). However, whether 

 the variation in size at maturity 

 noted in the above studies reflected 

 age differences or differences in in- 

 dividual growth rate could not be 

 determined because individual age 

 was not known. 



Mangold ( 1983) was able to docu- 

 ment not only variation in size at 

 maturity but also variation in age 

 at maturity in the octopus Eledone 

 moschata by raising known-age sib- 

 lings from hatching to maturity. She 



found marked differences in sibling 

 growth rates, some siblings show- 

 ing a tenfold difference in weight 

 after two months. She was also able 

 to document that the differences in 

 size at maturity resulted from some 

 individuals delaying maturation to 

 a greater size and age. Using sta- 

 tolith ageing techniques, Jackson 

 (1993) was able to delineate sea- 

 sonal differences in trends in gonad 

 growth and age at maturity for field 

 populations of Loligo ehinensis 1 and 

 Idiosepius pygmaeus. 



By following the growth of cepha- 

 lopods of known age in captivity, it 

 is possible to determine whether 

 variation in size at maturity is at- 

 tributed to differences in individual 

 age or to differences in individual 

 growth and maturity rates or to a 

 combination of both of these factors. 



It is now becoming more common to refer 

 to Loligo in Australia as Photololigo (see 

 Yeatman and Benzie, 1994). The species 

 previously referred to as Loligo ehinensis 

 (Jackson, 1990b, 1993, 1994; Jackson and 

 Choat, 1992 ) is now known to be a distinct 

 species inhabiting shallow water in tropi- 

 cal Australia and has been referred to as 

 Photololigo sp. 3 l Yeatman and Benzie, 

 1994) and Photolohgo cf. ehinensis least 

 coast form) in Dunning et al. (1994). Be- 

 cause this species has not yet been named, 

 and to avoid confusion, this species will be 

 referred to by its previously published 

 name. 



